1. Field of the Invention
The present invention relates to a phase change optical recording medium on which materials to be recorded are optically changed by irradiating electromagnetic wave, and which enables recording and reproducing and rewriting.
2. Description of the Related Art
As one of optical recording media which enables recording and reproducing and erasing by irradiating a light, there is known a so-called phase change optical disk. Such phase change optical disk utilizes a phase transition between a crystalline phase and an amorphous phase, or between a crystalline phase and other crystalline phase. Since overwriting on the phase change optical disk can be carried out by a single beam, and because of the simple optical system of driving, such optical disk is generally employed as AV-related recording media or as recording media for a computer.
Materials used for the recording media include Ge—Te, Ge—Te—Se, In—Sb, Ga—Sb, Ge—Sb—Te, and Ag—In—Sb—Te. In particular, Ag—In—Sb—Te is employed as a mark edge recording material, having features such as high sensitivity, and clear outline of amorphous part in a recording mark (see Japanese Patent Application Laid-Open (JP-A) No. 03-231889, JP-A No. 04-191089, JP-A No. 04-232779, JP-A No. 04-267192, and JP-A No. 05-345478, or the like).
However, these recording materials are to be used for an optical recording medium having relatively low recording density, such as CD-RW (Compact Disk-Rewritable). If it is used in DVD (Digital Versatile Disk) RAM or DVD-RW and the like, overwriting can be carried out when a recording linear velocity is around 3.5 m/s (×1 speed). However, when it becomes ×2 speed or faster, there is a problem that overwriting property may deteriorate. It is because speed for crystallization of the recording materials as described above is low, thus overwriting in high linear velocity becomes difficult.
It is possible to crystallize faster, by increasing the amount of Sb, but this may lead to fall in crystallization temperature due to increase of Sb, and then a storage property may be deteriorated.
To solve this problem, there is disclosed an approach to use Ag—In—Ge—Sb—Te system recording material in JP-A No. 2000-322740. It is possible to apply this approach when the recording linear velocity is 3.0 m/s to 20 m/s, but at a higher velocity, such as 20 m/s or more, it is not applicable.
In the meantime, GaSb has been proposed as a high-speed crystallization material (“Phase-change optical data storage in GaSb” Applied optics./vol. 26, No. 22115, November 1987). It is reported that as for this alloy, crystallization speed is extremely high, however, since a crystallization temperature is as high as 350° C., crystallization at the initial stage is difficult. Furthermore, the U.S. Pat. Nos. 4,818,666 and 5,072,423 disclose that Mo, W, Ta, Ni, Pd, Pt, Cu, Ag, Au, Zn, Cd, Al, Tl, Si, Ge, Sn, Pb, As, Bi, S, Se, Te or the like are added to GaSb, so as to enhance its property. However, such disclosures do not satisfy an overwriting property at high speed recording, degree of modulation and archival stability at the same time. For instance, the optical recording medium disclosed in the U.S. Pat. No. 4,818,666 utilizes the phase change between the crystalline and crystalline phases for optical recording. The modulation is 0.29 at the best, which causes problems in the practical use. The U.S. Pat. No. 4,818,666 states that a layer or a film lifts up, which is presumably because of the bubble generated on a portion where a laser beam is irradiated, when a content of Ga is less than 20%. It also states that, because of the lift-up, the level, in which the reflectance differs, becomes unstable. The U.S. Pat. No. 4,818,666 states that the invention stated therein causes problems in practical use, accordingly. The phase change between the crystal and crystalline phases utilizes the difference in reflectance derived from the difference in the crystal particle diameters. Therefore, it is unfavorable for the high-density recording that requires fine marks. The optical recording medium disclosed in the U.S. Pat. No. 4,818,666 does not allow the same recording capacity as DVD-ROM. Appl. Phys. Lett. 60 (25), 22 Jun. 1992, pp. 3123-3125 states an optical recording medium that utilizes a GeSb thin film and that is capable of phase change at an extra-high speed. An electron diffraction shown in the FIG. 1 attached thereto does not describe the orientation of crystals. Furthermore, the modulation between a phase which serves as the crystalline phase and the amorphous phase is 0.15 to 0.2, which causes problems in practical use.
Japanese Patent Application Laid-Open (JP-A) No. 2001-39031 discloses an optical recording medium whose recording layer is mainly formed of alloy as (SbxGe1−x)1−yIny (herein, 0.65≦x≦0.95, 0≦y≦0.2). However, there is only one statement in initialization condition of the optical recording medium. It only states that the initialization condition is to crystallize at an initial phase with a laser power density of around 2.6 mW/μm2. There is no statement about a high laser power density at an initial phase. The low laser power density as disclosed therein provides a medium having poorly oriented crystals and low reflectance. The recording linear velocity disclosed therein is as slow as 2.4 m/s to 9.6 m/s, which does not meet the present invention's a high recording linear velocity.
As described above, various kinds of phase change recording materials have been reported so far, but any of the materials has not been satisfactorily provided with the properties required by a rewritable phase change optical recoding medium. In particular, those have not been the materials such as having a high-density recording capacity equivalent of that of DVD-ROM's, being capable of handling a situation when recording linear velocity is further increased (−35 m/S), and simultaneously satisfying requirements as to overwriting property, degree of modulation, and archival stability.